Power converters are required to provide an output power supply with first characteristics from an input power supply with different characteristics —typically a high voltage AC supply is transformed to a lower voltage DC supply.
In some applications, such as data centers, minimum “hold up” times are required so that during input power supply discontinuities power continues to be supplied at the output of a converter.
Cycloconverters are very efficient but have minimal charge storage capability,
One approach to achieve hold up is to provide storage capacitors on the DC output of a converter. This requires, large storage capacitors to provide the required hold up as the stored charge cannot be utilized below a certain voltage level making this solution bulky and expensive.
U.S. Pat. No. 7,034,413 discloses a DC side hold up extension circuit in which a capacitor charged from a DC bridge is normally isolated from the DC/DC converter but can be switched in during loss of power supply. This approach also has limited. utilization of the charge stored in the storage capacitor.
U.S. Pat. No. 7,034,413 discloses a circuit in which a rechargeable battery is used to provide. hold up during loss of power supply. Batteries suffer from a slow recharge rate and require periodic replacement. Batteries also require specific charging circuits to ensure they are correctly charged.
Another approach disclosed in “Range winding for wide input range front end dc/dc converter,” by B. Yang, P. Xu, and F. C. Lee, in Proc. IEEE Applied Power Electron. Conf., 2001, pp. 476-479 is to switch the output of a converter between input windings of an output transformer having different numbers of turns. This approach results in abrupt output voltage changes when switching between windings.
U.S. Pat. No. 6,504,497 discloses a circuit employing an additional DC/DC boost-type converter utilized when hold up is required. This approach requires additional components, increasing cost, and an additional energy storage inductor having a detrimental effect on power density.
Two stage converters may have sufficiently large storage capacitors to provide required hold up times but they are inefficient due to losses or the two stages and require large storage capacitors to provide the necessary hold up. “Hold-up time extension circuit with integrated magnetics”, by Y. Sang, M. Jovanovic and D. L. Dillman, IEEE Applied Power Electronics Conference Record, pp. 219-225, 2005 is an example of a two stage converter using a boost output stage that operates in a different mode during loss of power supply to provide improved hold up time. The additional power stage and control components results in increased cost and a loss of efficiency.
It would be desirable to provide a power converter providing required hold up that is efficient, compact and inexpensive or which at least provides a useful choice.